Chemical additives for lubricating oils are used to control the physical and chemical properties of lubricating oils. These additives are used to modify oil viscosity and viscosity index, to make the oils more resistant to oxidation, and to keep engines and other mechanical equipment clean and protected against corrosion and wear. Water-soluble additives are also commonly used in applications ranging from aqueous hydraulic fluids to household cleaners and cosmetics.
Hydrocarbon-based chemical additives are designed for specific functions by choosing a hydrocarbon type and molecular weight range or molecular weight distribution to allow the additives to function in the fluid type of interest. For instance, high molecular weight polymers can be used to increase viscosity and viscosity index of mineral oils or synthetic oils. Water soluble polymers of polar compounds can be used to thicken water, or even allow water to be pumped more easily. Polar head groups can be designed to be attached to low or high molecular weight hydrocarbon tails to achieve detergency, dispersancy, antiwear or anticorrosion performance.
The hydrocarbon tail can be derived from natural fats or oils, or from petroleum fractions. Synthetic tails can be assembled by the polymerization of olefins or functionalized olefins or by poly-condensation of difunctionalized olefins or saturated compounds.
The patent literature frequently describes the use of polymers of olefins having 2 to 6 carbon atoms for use as oil-soluble tails suitable for use in making oil additives. Indeed, some patents refer to use of polymers of even longer chain olefin monomers for this purpose. Extensive use is made of ethylene and butene or iso-butylene oligomers in forming oil additives. High molecular ethylene-propylene olefin copolymers are commonly used to increase the viscosity index of lubricating oils. Propylene trimer and tetramer have been used as low molecular weight tails, and technology to make branch C.sub.20 to C.sub.100 polypropylene has been developed.
Despite the vast amount of work conducted heretofore, a need exists for novel ashless dispersants that have enhanced thermal stability and/or that can enable use of smaller amounts of viscosity index improvers in formulating finished lubricants, giving a cost reduction. Because of the relatively high temperatures to which finished lubricating oils are exposed during actual service conditions, improved thermal stability is a desirable property in ashless dispersants. The advantage of having an ashless dispersant which contributes viscosity increase to the lubricant and thus reduces the amount of viscosity index improver needed in the finished oil is referred to, for example, in U.S. Pat. No. 4,234,435.